This invention relates to amino sulfate derivatives of epoxy resins capable of uniformly dispersing in an aqueous medium. It also relates to uses of said amino sulfonate derivatives as an ingredient of water based coating compositions.
Epoxy resins have been used in various fields because of their excellent adhesive properties, high dielectric properties and the like. Normally, they are used as such or as a solution in an organic solvent. Since the resins themselves are generally hardly soluble in water, water based epoxy resin emulsions are prepared by emulsifying the resin in water with the aid of conventional surfactants. Epoxy resin emulsions, however, suffer from certain disadvantages that the resulting film does not have sufficiently strong mechanical, water-proof, anti-chemical and other characteristics.
Another approach to produce water based epoxy resins is to introduce a hydrophilic group into the molecule. To this end, Japanese Unexamined Patent Publication No. 55-3463 discloses the preparation of a water based epoxy resin by reacting the resin with an amino carboxylic acid to cleave the oxirane ring with a hydroxyl group being formed, and then esterifying the hydroxyl group with a dicarboxylic acid anhydride to form a hemi-ester having a pendant carboxyl group. This process requires two steps with two different reactants for cleaving the oxirane ring and esterifying the resultant hydroxy group, respectively. Furthermore, introduction of a great number of such pendant carboxyl groups per mole is required for epoxy resins having a hodrophobic backbone such as bisphenol type to impart a high acid number to the resin. This is not satisfactory because the resultant films are not sufficiently water-resistant.
It is, therefore, an object of the present invention to provide a water based epoxy resin composition free of abovementioned defects.
It is another object of the present invention to provide a water based epoxy resin composition having an amphoteric amino sulfonate terminal group, which is easy to prepare by a one-step reaction with a single reactant, and which retains most of benefical properties of the starting epoxy resin.
Other objects and advantages of the present invention will become apparent as the description proceeds.